my personal opinions are the lying hurts me more. I can understand being in a relationship with someone but I can also understand desires and urge to pursue another. I can deal with being in a committed relationship in the sense that we would come home to each other and no matter what we'd always be together. If you were only looking for s*x with another then thats something I can accept if youre honest about it and willing to accept that i may also have the same urges. But if youre sneaking around lying about where you are and trying to hide it then thats different. You wanna fool around a little on the side, ok tell me. But dont go off planning a life with another all behind my back.
Cheating breaks trust all at once. Lies do it little by little. If a partner lies, you'll get over it. If a partner cheats, that influences your relationship from then until you meet someone so completely trustworthy that somehow, you're okay again.